This invention generally relates to automatic methods for ensuring stability of a control system for which the direction and magnitude of the response to a command input is unknown or uncertain.
As used herein, the term “control system” means a device or set of devices that manage, command, direct or regulate the behavior of other devices, equipment or systems (hereinafter “plant”). Although the embodiment disclosed herein has particular application to control of the rotation of a spacecraft about an axis, the control method disclosed and claimed herein has application to plant other than spacecraft, such as electric motors or servomechanisms.
Adaptive control offers excellent robustness to errors in control actuator magnitude and misalignment. These errors can stem from a host of sources such as mischaracterization of the control actuators, misalignment during assembly, and failures or degradation in the hardware. Additionally, there have been developments on using adaptive control to provide robustness to errors in the sign on the control actuator.
While not common, there have been several space missions that have encountered such an error, which can be caused by incorrect mounting of the hardware or miswiring during assembly. Correcting for this error requires substantial ground intervention since the typical attitude control law will destabilize the closed loop system under a sign error.
In particular, there have been instances where a reaction wheel of a spacecraft has been miswired during assembly, causing the reaction wheel to respond in the opposite direction to the commanded direction. One solution to this problem is to perform detailed ground testing to verify correct installation of control actuators, such as reaction wheels, as well as to characterize their effectiveness. However, ground testing does not always detect a reversed actuator.
Another solution is to perform on-orbit testing by sending open-loop commands to the spacecraft to either verify the expected response and/or perform system identification using the response. On-orbit open-loop testing and identification requires ground interaction which is cost and schedule intensive. In one known instance, reaction wheel miswiring on a spacecraft required substantial ground intervention to correct.
Without ground intervention, a reversed reaction wheel will result in system instability and potential loss of the spacecraft. Therefore it would be advantageous to provide an autonomous feedback control system to adapt to such error, thereby safeguarding against a potentially catastrophic loss.
Thus, there is a need for an adaptive control method that provides the ability to address control uncertainties in both the sign and magnitude of the control power of the system to ensure stability and command tracking. The adaptive control system must also function correctly and remain internally stable in spite of control limits, e.g., maximum reaction wheel torque.